Novel electrochemical method for producing hydrogen, and device for implementing same

ABSTRACT

An electrolysis device including a bioanode and a cathode catalyst, a method for implementing the same, and the use thereof for producing hydrogen.

In the field of energy, the taking into account of the increase inneeds, in safety of supply, of environmental hazards requires still moreextensive research work to be developed on diversification and optimumuse of primary resources (fossil, nuclear, renewable resources . . . ).

Hydrogen, which allows flexible storage and distribution of energy whilebeing not very polluting, may be a carrier for supplying energy todifferent sectors of activities.

Hydrogen, the most abundant element on our planet, is however notdirectly available in nature. In order to be economically andecologically viable, massive use of hydrogen as a source of energy(thermal and electrical energy) depends on the development of all theindustrial steps from production to final use, via storage anddistribution.

At the production level, three criteria have to be observed:

-   -   competitiveness: investment and production costs should not be        too high;    -   energy yield: the energy consumption for production should be        limited;    -   cleanliness: the method should not be polluting in order to keep        one of the major assets of hydrogen.

Presently, 95% of the hydrogen (essentially used in the chemical andpetrochemical industry) is produced by reforming from fossil fuels. Thismethod does not meet all the criteria: the price cost is three timeshigher than that of natural gas and there is emission of “bad” CO₂ fromfossil compounds. The other methods are related to thermochemical orelectrochemical decomposition of water and to direct production from thebiomass. Hydrogen may also be obtained as a byproduct of cracking, steamcracking, catalytic reforming units, electrolysis of brines, coking.

Today, the production of hydrogen via an electrochemical route (waterelectrolysis) represents 4% of the total production (Alain Damien,Hydrogène par électrolyse de l'eau (Hydrogen by electrolysis of water),Techniques de l'Ingénieur, volume Génie des Procédés, J6 366) and ismainly conducted according to two techniques:

-   -   at atmospheric pressure followed by compression required for        storage and transport;    -   at high pressure: less than or equal to 30 bars for industrial        apparatuses and which may exceed 10 bars on certain versions        intended for submarines.

Electrolysis of water in an alkaline medium (essentially potassiumhydroxide in a concentration from 25% to 40% by mass) benefits from along experience. Its development is based on the development of novelmaterials meeting several criteria: resistance to corrosion in thisalkaline medium and catalysis of the electrode reactions (high currentdensity and low overvoltage). At the cathode, the deposits based onnickel on an iron base are the most used. Presently novel research workis conducted in order to show that materials of less high quality may beused as a cathode and allow a reduction in the investment costs.(Marcelo et al., International Journal of Hydrogen Energy 33 (2008)3041-3044; Olivares-Ramirez et al., International Journal of HydrogenEnergy 32 (2007) 3170-3173). At the anode: the base should be more noble(nickeled steel or massive nickel) and the deposition of the catalyststill remains a delicate point which is the subject of much researchwork.

Industrial application of electrolysis in an acid medium deals with theproduction of small amounts of very pure hydrogen for laboratories. Themain characteristic lies in the use of a cationic membrane (Nafion type)and of catalysts based on precious metals for the cathode (platinumblack) and the anode.

Electrolysis of water using PEM (Proton Exchange Membrane) technology isalso developed and achieved by feeding the anode with pure water: inorder to increase the energy yield, metal catalysts (Pt, Pd) are used inthe form of deposits on the faces of the membrane.

Work is also conducted for achieving electrolysis of steam. Hightemperature electrolysis is more efficient than the method at roomtemperature since a portion of the energy required for the reaction isprovided via heat, which is less expensive to obtain than electricity).

Electrolysis of water is an electrolytic process which decomposes waterinto oxygen and hydrogen gas by means of an electric current. At thecathode, a reduction reaction occurs with production of dihydrogen gasaccording to the reaction: 2H⁺+2e⁻→H₂ gas. At the anode, an oxidationreaction occurs: 2H₂O→O₂ gas+4H⁺+4e⁻. The overall reaction is:

2H₂O→O₂+2H₂.

Thus, the potential on the terminals of an electrochemical cell,achieving electrolysis of water, cannot be less than the thermodynamicvalue of 1.23 volt at standard temperature and pressure when the anodicand cathodic solutions are at the same pH.

The economic yield is related to the cost of the required electricity.80% of the price of hydrogen produced via an electrochemical route isdue to electricity consumption.

It is therefore desirable to reduce energy consumption of theelectrochemical process while reducing the aggressivity of the mediaused (potash at 25-30% corresponds to a pH of about 15) for operationunder mild pH conditions in the vicinity of neutrality.

The application FR 2 904 330 describes a device for electrolysis ofwater and its use for producing hydrogen, said device comprising ananodic and cathodic compartment, such that said cathodic compartment hasan electrolytic medium comprising at least one weak acid capable ofcatalyzing the reduction and an electrolytic solution, the pH of whichis comprised between 4 and 9.

Nevertheless, this device does not allow sufficient lowering of thepotential and therefore of the required electricity consumption.

Torres et al. (Applied Microbiology and Biotechnology 77, 3, 2007,689-697) describe electrolysis devices having only a graphite cathode;now such graphite cathodes do not have optimum characteristics forproducing hydrogen by reduction of water.

Micro-organisms may spontaneously adhere on any types of surfaces andform films called biofilms consisting of said micro-organisms, of amatrix of exopolymeric substances (polysaccharides, proteins,macromolecules . . . ) which they excrete, or substances produced bymicrobial metabolisms and accumulated compounds from the medium or fromdegradation of the supporting surface. It was recently discovered thatthe biofilms developed on conducting surfaces are capable of using thesesurfaces for discharging the electrons stemming from their metabolism(D. R. Bond et al., Science 295 (2002) 483, and L. M. Tender et al.,Nature Biotechnology 20 (2002) 821; H. J. Kim et al., Enzyme andMicrobial Technology 30 (2002) 145).

Other biofilms have been shown to be capable of catalyzing reduction ofoxygen on materials such as stainless steels (A. Berge) et al.,Electrochemistry Communications, 2005, 7, 900-904; FR 02 10009) which,in their initial condition without any biofilm, are not known forensuring high reduction rates of oxygen. These biofilms may be exploitedfor discharging from the colonized surface, the electrons of the systemtowards a dissolved compound, for example oxygen.

Whether they are capable of catalyzing electrochemical oxidation orreduction reactions, these biofilms will subsequently be calledelectrochemically active biofilms or EA biofilms.

These technologies have essentially been developed for battery cells,i.e. for producing electricity.

Recently, these microbial battery cells have been used for assistingwith the production of hydrogen from fermentation of organic materials,such as glucose (Liu et al., Environ. Sci. Technol. 2005, 39, 4317-4320;Call et al. Environ Sci. Technol. 2008, 42, 3401-3406; Rozendal et al.,International Journal of Hydrogen Energy 31 (2006) 1632-1640).

Nevertheless, all these technologies use platinum, notably as a depositon a carbon or TI base with a load from 0.35 to 5 mg Pt/cm² at thecathode in order to catalyze reduction of water. Because of theassociated cost, these systems can only be contemplated for veryspecific uses, for example when very pure hydrogen gas is desired.

The present inventors have henceforth developed a novel electrolyticdevice comprising a cathode made in a non-noble material, a weak acid asa catalyst of reduction of water and a bioanode in a non-noble material.Unexpectedly, the thereby generated device allows potentialization ofthe synthesis of hydrogen and reduction of electricity consumption tolevels much lower than those of existing technologies.

According to a first object, the present invention therefore relates toan enhanced electrolysis device intended to produce hydrogen by anelectrochemical process, by joint use of a bioanode and of cathodiccatalysis by weak acids.

Thus, the device according to the invention comprises:

-   -   a cathode immersed in an electrolytic solution comprising at        least one weak acid and the pH of which is comprised between 4        and 9;    -   an anode immersed in an electrolytic solution capable of forming        an electrochemically active biofilm at the surface of the anode,        and comprising at least one biodegradable organic compound and        capable of being oxidized at the anode.

The anode with an EA biofilm formed at its surface and/or in theelectrolytic solution capable of forming an EA biofilm is designatedhere by the term of “bioanode” or microbial anode.

The device according to the invention also comprises the followingpreferential embodiments, as well as any of their combinations:

-   -   Said electrolytic solution in which the cathode is immersed and        said electrolytic solution in which the anode is immersed, may        be identical or distinct. Thus, when they are identical, the        device consists of a single compartment; when they are distinct        (for example in the case of different pHs), the device consists        of two compartments, one anodic compartment comprising said        anodic electrolytic solution in which the anode is immersed, and        the other cathodic one comprising said cathodic electrolytic        solution in which the cathode is immersed, said anodic and        cathodic compartments then being separated by a separator        element allowing migration of the irons between said anodic and        cathodic compartments.    -   The material of the anode is selected from any conducting        material on which biofilms may be formed, such as notably        carbon, graphite, stainless steel, nickel, platinum, DSA . . . ,        and which may optionally be modified by catalytic deposition        customarily used for improving conductivity (platinum,        palladium, etc.).    -   The cathode is a conducting material or comprises such a        conducting material, notably at least its surface is in such a        conducting material or comprises it. This material is selected        from any conducting material, preferably distinct from carbon        and from all its forms, notably selected from the group formed        by conducting polymers, either oxidized forms or not of Fe, Cr,        Ni or Mo, and their different alloys, notably stainless steel,        notably 304L, 316L, 254 SMO steels. By stainless steel is meant        alloys comprising iron, nickel and chromium, comprising more        than 12% chromium, notably selected from (chemical analysis in %        by weight):

304L: C, 0.02%, Cr: 17-19%, Ni: 9-11%;

316L: C, 0.02%, Cr: 16-18%, Ni: 11-13%, Mo (molybdenum): 2%,

430: C, 0.08%, Cr: 16-18%,

409: C, 0.06%, Cr: 11-13%, Ti (titanium),

254 SMO, etc. . . .

-   -   Said electrolytic solution in which the cathode is immersed        contains at least one weak acid capable of catalyzing the        reduction. Preferably, the pH is thus comprised between 7 and        8.5, more preferentially about 8;    -   As weak acids, those which have the acid form at the selected pH        of the solution in which the cathode is immersed, are preferred;        mention may notably be made of the different acido-basic species        of the phosphate, such as orthophosphoric acid,        dihydrogenphosphate, hydrogenphosphate, lactic acid, gluconic        acid and/or mixtures thereof. Dihydrogenphosphate or        hydrogenphosphate and more preferentially dihydrogenphosphate or        a mixture of hydrogenphosphate/dihydrogenphosphate are        preferred;    -   The concentration of said weak acid may be comprised between 0.1        and the solubility of said weak acid in the electrolytic        solution in which the cathode is immersed. Thus, in the case of        dihydrogenphosphate, the concentration is generally less than        1.5 mole per liter. More preferentially about 0.5 mole per        liter.    -   Advantageously, a hydrogenphosphate/dihydrogenphosphate mixture        is used for obtaining a buffer medium at a pH of about 8;    -   Said electrolytic solution in which the anode is immersed,        generally has a pH comprised between 4 and 9; preferentially,        the pH of the electrolytic solution in which the anode is        immersed, is identical with that of the electrolytic solution in        which the cathode is immersed, more preferentially the pH is        neutral, comprised between 6 and 8.    -   The reactions at the anode and at the cathode may be conducted        at a close pH; preferentially equal to about 7; also, it is        possible to do without any separator element between both        compartments. Nevertheless, if required, such a separator        element may separate both compartments; said separator element        may then be an electrochemical bridge known to one skilled in        the art, such as a proton exchange membrane (PEM), a cationic        membrane, a ceramic, an outer filtration membrane (UF), an anion        exchange membrane (AEM), a bipolar membrane, or further a simple        polymeric separator or any other separation allowing ion        conduction, known to one skilled in the art; as an illustration        mention may thus be made of the commercial membranes Nafion® 117        or 1135, or CM1-7000S.    -   Said electrolytic solution in which the anode is immersed        advantageously comprises one or more micro-organisms or        consortia of micro-organisms stemming from natural media (water        effluents, muds, compost, sediments . . . ), known to form EA        biofilms at the surface of the anode (Du et al, Biotechnology        Advances 25 (2007), pp. 464-482, Rozendal et al, Trends in        Biotechnology 26 (2008), pp. 450-459); thus, this is the case        for example of micro-organisms present in marine sediments. In        an original way, no electrolysis cell had not yet applied marine        sediments as an electrolytic solution at the anode for producing        dihydrogen. These marine bioanodes have two advantages: i) they        give the possibility of operating at high salinities, which is        very beneficial since the internal resistance of the        electrolyzer is thus reduced (the anodes formed from effluents        are known to not accept strong salinities, which increases the        internal resistance to the cells) and ii) the fact of operating        at high salinities may give the possibility of reducing        parasitic methanogenesis reactions, notably since the        micro-organisms producing methane do not appreciate strong        salinities.    -   Said electrolytic solution in which the anode is immersed,        advantageously contains marine sediments, and is preferentially        seawater or an electrolytic solution of similar salinity;    -   As a biodegradable organic compound capable of being oxidized at        the anode, mention may be made of any natural or synthetic        organic compound, derived from biomass for example or as an        illustration, such as acetic acid or an acetate, such as sodium        acetate. The term of biodegradable indicates that the organic        compound is capable of being transformed by micro-organisms.        Advantageously, the organic compound may be selected from the        weak acids described above.

The device according to the invention associating catalysis of thereduction of water by weak acids, an anodic oxidation reaction oforganic materials by micro-organisms gives the possibility of obtaininga theoretical potential of the electrolysis cell of 0.11 volt at pH 7,in the case of the acetate, and therefore much smaller than thepotential of 1.23 V required by electrolysis of water.

Said device may notably be an electrolyzer.

Said device may assume diverse geometries, notably depending on the sizeof the installation. The geometry of the device is not critical and maybe optimized by one skilled in the art according to techniques known perse.

Generally, the anodic and cathodic compartments are separated by one ormore partition membranes, ensuring the passage for ions betweenelectrolytic solutions. Thus, it is possible to mention the embodimentaccording to which the cathodic compartment is a tube plunging into theelectrolytic solution of the anodic compartment and wherein one or moremembranes are laid out either in the base of the cathodic tube, and/orin the side walls of the cathode tube immersed in the electrolyticsolution of the anodic compartment. The designs 1 and 2 of the exampleswhich follow, illustrate these embodiments.

Advantageously, the shape and the structure of the system according tothe invention may be designed so as to generate exchange surface areasas large as possible for each of the functional areas. Mention maynotably be made of porous structures, of the foam or felt type, or ofany type of structure with a large specific surface area or a highdegree of voids known in the state of the art. Also, shapes of thehelix, dendrite, grid type etc. which increase the surface area of eachelement for a given volume may be favorable to its effectiveness. Theshape may also be designed in correlation with the hydrodynamics of themedium for the circulating or stirred liquid environments. The geometryof the anode may be optimized depending on known constraints ofelectro-microbial catalyses, in particular for promoting adhesion ofbacterial cells and electron transfer between these cells and theelectrode. The cathode may be optimized for promoting gas evolvement andrapid discharge of the bubbles. On this point, it will be possible tobenefit from different developed technologies for the design ofelectrodes with gas evolvement.

Said device may also further comprise any element customarily used ininstallations for electrolysis of water in an alkaline medium, such asfor example means for collecting the thereby formed hydrogen such as aflow meter, a valve, a compressor and/or one or more referenceelectrodes such as the calomel-saturated electrode (CSE), ammeters,voltmeters, a voltage or current generator, etc.

Said micro-organism(s) forming an EA biofilm at the surface of the anodemay spontaneously exist in the electrolytic solution of the anode.Alternatively or cumulatively, sowing the electrolytic solution withsuitable micro-organism(s) in any possible forms (inocula, culturebroths, lyophilizates, etc) may be contemplated. For this, it ispossible to use as an inoculum, samples of media known to containmicro-organisms which easily form EA biofilms, such as muds of aqueouseffluents (sewage works for example), sediments or biofilms for examplemarine sediments, composts, pure cultures of micro-organisms or anyother medium known to one skilled in the art for giving EA biofilms.Advantage may be taken by sowing with samples of EA biofilms collectedearlier on anodes of any system applying EA biofilms, such as microbialfuel cells for example. It is actually known that EA biofilms are goodinocula for reforming EA biofilms. The first subcultures often ensure asignificant increase in the catalytic activity. It is also possible touse pure cultures of micro organisms known for their capacity of formingEA biofilms, such as Geobacter, Desulfuromonas, Shewanella,Geopsychrobacter, Rhodoferrax, Geothrix, etc., and any EA strain knownin the state of the art.

The sowing with one or several inocula may be carried out upon startingoperation of the device, it may also optionally be renewed duringoperation in order to reactivate the device for example, for example inorder to compensate for a reduction in its efficiency or after anoperational incident.

The present invention also relates to the electrochemical process forproducing hydrogen, by means of the device according to the invention.Thus, the process comprises the passing of a current into a deviceaccording to the invention, such that the anode and the cathode areconnected to the opposite terminals of a current or potential source.For this purpose, any device may be used which allows delivery of acurrent or the maintaining of a potential difference, for example apotentiostat, a stabilized power supply or a photovoltaic panel.Generally, the potential range is comprised between 0.1 and 5V, notablybetween 0.2 and 2 V.

The intensity notably depends on the desired production and on thesurface area of the electrodes.

Said method may be applied in a wide range of temperature and pressureoperating conditions; operating at a temperature comprised between roomtemperature and 80° C. and at a pressure comprised between atmosphericpressure and 30 bars is notably preferred.

The water used for the cathodic electrolytic solution should notnecessarily observe any particular purity criteria; thus, the water ofthe electrolytic solution may be seawater or distilled water, from themoment that the weak acid used and optionally electrolyte ion supports(such as KCl, NaCl, etc. . . . ) are in a sufficient amount in order toensure conductivity of the reaction medium.

The method according to the invention may also comprise the preliminarybiasing step by applying an imposed current or by maintaining a voltageof either one or both of the electrodes at an imposed potential,corresponding to the theoretical thermodynamic potential of theoxidation and/or reduction reaction to be initiated.

There again, these elements are within the capacity of one skilled inthe art.

The present invention also relates to an installation, notably forproducing hydrogen, comprising a device according to the invention.

The present invention, allowing production of hydrogen via a lowtemperature electrochemical route is therefore distinguished by:

-   -   low electricity consumption,    -   operation under milder pH conditions than with the use of potash        (present industrial process),    -   the use of a homogeneous catalyst (weak acids) for the cathode        reaction which allows replacement of the noble metals        customarily used for the cathode (Pt, Ni used in present        processes) with less noble metals such as steels, and    -   the use of the biomass for lowering the potential of the anodic        reaction.

Other objects, features and advantages of the invention will becomebetter apparent upon reading the following description, given as anillustration and not as a limitation.

FIGURES

FIG. 1 schematically illustrates a device according to the inventionwherein (1) illustrates the cathodic compartment, (2) illustrates apermeable membrane, (3) illustrates the anode, (4) illustrates a devicefor measuring the flow rate of the hydrogen formed and (5) illustrates avalve.

FIG. 2 illustrates the production of hydrogen depending on the voltageon the terminals of the electrolysis cell (design 1)

FIG. 3 illustrates the biasing curves plotted from the results obtainedupon biasing the electrodes with Ecell varying from 0 to 2V. For eachEcell, a current is obtained; further the cathodic potential is measuredrelatively to a reference electrode, the anodic potential is theninferred therefrom (Ecell−Ecath).

FIG. 4 a illustrates the energy consumption versus the productioncapacity for the devices according to the invention and those of theprior art.

FIG. 4 b is a zoom of FIG. 4 a for low productions.

EXAMPLES

1. The Experimental Set-Up

The experiments were conducted in an electrochemical reactor includingtwo distinct compartments as described below:

Cathodic Compartment where Hydrogen Production Occurs

A tube in Plexiglas with a diameter of 6 cm with a height of liquid of30 cm, i.e. a cathodic volume of 0.85 L. The cathode consists of twoplates of stainless steel 254 SMO corresponding to a total geometricalsurface area of 50 cm², it is immersed into the cathodic mediumconsisting of phosphate buffer (0.5M pH 8).

Two compartment designs were tested:

Design 1: a membrane (Nafion® 117) is attached to the bottom of the tubedefining an exchange surface area of 28 cm² (diameter: 6 cm).

Design 2: a membrane (CM1-7000S—Membranes International Inc. USA) isattached in 3 rectangular windows (20×1.5 cm²) dug at the periphery ofthe bottom of the tube defining a total exchange surface area of 90 cm²(20*1.5 cm).

The upper end of the tube is hermetically sealed and the produced gas iscollected via a bubble flowmeter in order to evaluate the production ofhydrogen.

Anodic Compartment where Oxidation of the Acetate Catalysed by a MarineEA Biofilm Occurs

A tube in Plexiglas with a diameter of 20 with a height of liquid of 80cm, i.e. an anodic volume of 25 L.

The anode is carbon felt (Carbone Lorraine) with a geometrical surfacearea of 2,500 cm² (¼ m²) immersed in natural seawater (pH 7) in whichsodium acetate (2 g/L) was dissolved.

The bioanode of this compartment was prepared by using an inoculumconsisting of marine biofilm harvested at La Tremblade (Atlantic Ocean).The carbon electrode was biased at −0.2 V/CSE for 15 days during whichthe current strongly increased (up to 0.4 A/m²) demonstrating theefficiency of the bioanode for oxidation of the acetate.

The end of the cathodic compartment including the membrane is immersedin the anodic compartment. A potential difference, designated by Ecell(potential on the terminals of the electrolysis cell), is imposedbetween both electrodes via a potentiostat (VMP2 from Biologic) or asolar panel coupled with a resistor of variable value and the outputcurrent is measured. A reference electrode immersed in the cathodiccompartment is also used for tracking the performances of bothelectrodes (measurement of the potential of the cathode and deduction ofthat of the anode).

2. The Results of Design 1+Potentiostat

The results obtained with the cathodic compartment according to design1, i.e. a membrane at the bottom of the cathodic tube, are grouped inTable 1.

TABLE 1 design 1 Energy Current H₂ Energy consumption Ecell I in densityproduction Faradic consumption kWh/ (V) mA (A/m²) (mL/h) yield kWh/m³ H₂Nm³ H₂ 0.0 0.0 0.0 0 — — — 0.2 2.5 0.5 Visible — — — 0.4 5.0 1.0 Visible— — — 0.6 14.0 2.8 4.8 74 1.8 1.9 0.8 29.0 5.8 11.4 85 2.0 2.3 1.0 44.08.8 19.8/9.9* 97 2.2 2.5 1.2 59.0 11.8 31.2 114 2.3 2.5 1.4 75.0 15.042.0 121 2.5 2.8 1.6 91.0 18.2 49.2 117 3.0 3.3 1.8 107.0 21.4 54.6 1103.5 3.9 2.0 125.0 25.0 60.0 104 4.2 4.6 *value for a surface areareduced to 25 cm²

FIG. 2 illustrates the production of hydrogen versus the potential onthe terminals of the electrolysis cell (Ecell). Finally, the biasingcurves were plotted for observing the individual performances of each ofthe electrodes (FIG. 3).

These first results show that it is possible to produce hydrogen in anelectrochemical cell by applying very low potentials between the cathodeand the anode and this in a medium with a neutral pH (7-8): a current isdetected for a potential of 0.2V for which low gas evolvement isobserved. Next, when the potential of the cell increases (Table 1 andFIG. 2), the production of hydrogen increases.

As a comparison, for a production of the order of 20 mL/h (correspondingto 10 mL/h for a surface area reduced to 25 cm²) i.e. an output currentof 8.8 A/m², a cell potential of 1.0V is needed, while 1.90V is neededfor operation in a concentrated potash medium and 1.67V for anelectrolysis cell including a cathodic compartment containing phosphate(0.5M) and an anodic compartment containing concentrated potash (asdescribed in FR 2 904 330 (Table 4)).

A reduction by more than 40% of the required potential is thereforeobserved, i.e. a reduction by 49% of the electricity consumption ascompared with operation in a potash medium, by 40% as compared with thephosphate system with the cathode combined with the potash at the anode.

During the test, the potential of the cathode varies from −0.76 to −2.09V/CSE while the potential of the anode varies from −0.56 to −0.09 V/CSEwhen the output current passes from 2.5 to 125 mA (FIG. 3): thetime-dependent change of the potential of the anode shows that thisbioanode is capable of providing a lot of current and is not thelimiting factor. On the cathode side, the potential extends over agreater range, and the faradic yield is smaller at lower Ecell valuesand reaches (or even exceeds) 100% for Ecell values greater than orequal to 1.2V. In fact for Ecell below 1.2V, the current is relativelylow and the proportion of the residual current (which would be obtainedwithout any phosphate in the solution) over the total current is moresignificant. Yields greater than 100% may perhaps be explained bydiffusion into the cathodic compartment of the carbon dioxide formed atthe anode during oxidation of the acetate, which may notably reduce thepurity of the collected hydrogen or modify the composition of thecollected gas.

3. The Results of Design 2+Potentiostat

The results obtained with the cathodic compartment according to design2, i.e. a membrane adhered to the three windows at the bottom of thecathodic tube are grouped in Tables 2 and 3: two cathodic solutions weretested, one consisting of seawater+phosphate (0.5M, pH 8) and the otherone consisting of phosphate (0.5M, pH 8) dissolved in distilled water.With respect to the results obtained with design 1, the deliveredcurrents for a given cell potential are still higher with this noveldesign (cf. tables and biasing curves, FIG. 3): a production ofrespectively 3.6 and 7.4 mL/h in seawater and distilled water isobtained from a cell potential of 0.4V. On the biasing curves (FIG. 3),it may be noticed that the anodic curves are very similar and that thedesign modification has consequences especially on the cathodic branch:the larger membrane surface area allows better distribution of the fieldlines and therefore an increase in the active surface area of thecathode. Moreover, modification of the cathodic medium (seawater ordistilled water) entails significant differences: the increase inconductivity is actually far from having the expected effect—less lossesvia ohmic drop, therefore increased production efficiency—on thecontrary, it would seem that the numerous constituents of seawater arean obstacle to the reduction of the hydrogen atoms of the phosphatespecies and therefore to the production of hydrogen (absorption/reactioncompetition). Pollution of the membrane may also be imagined, by the Na+ions which would block the ion exchange sites of the membrane and wouldinduce an ohmic drop reducing the performances of the system.

TABLE 2 design 2 seawater Current H₂ pro- Energy Energy Ecell I indensity duction consumption Consumption (V) mA (A/m²) (mL/h)* kWh/m³ ofH₂ kWh/Nm³ of H₂ 0.0 0.0 0.0 0.0 — — 0.2 1.5 0.3 visible — — 0.4 10.52.1 3.6 1.2 1.3 0.6 43.0 8.6 19.4 1.3 1.5 0.8 80.0 16.0 37.1 1.7 1.9 1.0115.0 23.0 53.3 2.2 2.4 1.2 125.0 25.0 58.0 2.6 2.9 1.4 170.0 34.0 78.83.0 3.4 1.6 200.0 40.0 92.7 3.5 3.8 1.8 276.0 55.2 128.0 3.9 4.3 2.0358.0 71.6 166.0 4.3 4.8 *Evaluated from measurements obtained withdesign 1 for I < 80 mA, for I > 80 mA, faradic yield taken to be equalto 100%.

TABLE 3 design 2 distilled water Current H₂ pro- Energy Energy Ecell Iin density duction consumption consumption (V) mA (A/m²) (mL/h)* kWh/m³of H₂ kWh/Nm³ of H₂ 0.0 0.0 0.0 0 — — 0.2 1.5 0.3 Visible — — 0.4 16.03.2 7.4 1.2 1.3 0.6 63.0 12.6 29.2 1.1 1.3 0.8 112.0 22.4 51.9 1.7 1.91.0 160.0 32.0 74.2 2.2 2.4 1.2 207.0 41.4 96.0 2.6 2.9 1.4 250.0 50.0115.9 3.0 3.4 1.6 302.0 60.4 140.0 3.5 3.8 1.8 357.0 71.4 165.6 3.9 4.32.0 410.0 82.0 190.1 4.3 4.8 *Evaluated from measurements obtained withdesign 1 for I < 80 mA, for I > 80 mA, faradic yield taken equal to100%.

4. The Results of Design 2+Coupling of Renewable Energy

Finally, tests were conducted by coupling the reactor provided with thecathodic compartment of design 2 (distilled water+0.5M phosphate, pH 8,cathode with a geometrical surface area of 25 cm²) with a photovoltaicpanel. The voltage delivered by the panel to the electrochemical cellwas regulated by introducing into the circuit a variable resistor (1 to10Ω).

The results, grouped in Table 4, are by far the best in terms ofproduction: 11 mL/h of hydrogen are produced with a cell voltage of 0.4V(bold line) i.e. a 0.6V reduction of the required potential, as comparedwith design 1 which, as a reminder, already allowed division by 1.7 (andmore) of the potential required during electrolysis in a cathodic potashor potash+phosphate medium.

TABLE 4 design 2 + photovoltaic Current H₂ pro- Energy Energy Ecell I indensity duction consumption consumption (V) mA (A/m²) (mL/h)* kWh/m³ ofH₂ kWh/Nm³ of H₂ 0.00 0 0.0 0.0 — — 0.41 27 10.8 10.6 1.0 1.2 0.81 11144.4 51.5 1.7 1.9 1.12 166 66.4 77.0 2.4 2.7 1.73 383 153.2 177.6 3.74.1 2.20 530 212.0 245.8 4.7 5.3

5. Comparative Results

In terms of energy consumption per Nm³ of produced hydrogen (N=Normalconditions=273 K), the gain is highly significant. For a production ofthe order of 10-11 mL/h on a stainless steel cathode (geometricalsurface area 25 cm²), the following order is obtained from the mostenergy-consuming to the least energy-consuming (synthesis of Tables 1 to4):

Present Industrial Technique:

Cathodic potash pH 15 and anodic potash pH 14: 4.9 KWh/NM³H₂

FR2 904 330:

Cathodic phosphate (0.5M, pH8) and anodic potash pH15: 4.3 kWh/Nm³ H₂.

Device according to the invention:

-   -   Cathodic phosphate (0.5M, pH8) and anodic biodesign 1 pH7: 2.5        kWh/Nm³ H₂ (takes into account the geometrical surface area of        50 cm²)    -   Cathodic phosphate (0.5M, pH8) and anodic biodesign 2 pH7: 1.2        kWh/Nm³ H₂

The device according to the invention allows a reduction in the energyconsumption by more than 49% (design 1) and 75% (design 2) as comparedwith the industrial technique and by 40% (design 1) and by 72% (design2) as compared with that of FR 2904330.

The device according to the invention was also compared withtechnologies for producing hydrogen from organic materials, using acathode in a noble material, as described by Liu et al., Environ. Sci.Technol. 2005, 39, 4317-4320; Call et al., Environ. Sci. Technol. 2008,42, 3401-3406; Rozendal et al., International Journal of Hydrogen Energy31 (2006) 1632-1640.

It emerges therefrom that:

-   -   for a same cell potential over a range from 0 to 1.2V, the        productions of hydrogen reported by these authors are all less        than those obtained according to the invention. Moreover,        according to the invention, the system may attain production        rates of the order of 0.1 m³/h/m² of cathode, therefore much        more significant than those reported which are all less than        0.005 m³/h/m² of cathode (see FIGS. 4 a and 4 b). Moreover, in        order to produce 0.1 m³/h/m², the energy consumption according        to the invention is of the order of 5 kWh/Nm³ of H₂. Under        conditions in a concentrated potash medium, this consumption        corresponds to a production rate per unit surface which is 20        times less.    -   Moreover, for the highest production rate reported in the        literature (5.2 10⁻³ m³/h/m², see Call et al., supra), the        energy consumption is 2 kWh/Nm³ of H₂; it passes to 1.3 kWh/Nm³        of H₂ by using the device according to the invention (steel        cathode and phosphate concentration of 0.5 M) i.e. a 35%        reduction.

This last result is to be compared with the 13% reduction (4.3 vs. 4.9kWh/Nm³ of H₂) obtained with the substitution with the steel cathode andthe presence of weak acid alone.

This result actually demonstrates the synergy of the bioanode combinedwith the weak acid catalysis at the cathode.

-   -   Finally, these authors work with a cathode containing platinum        unlike the device according to the invention which applies        cathodes in stainless steel, an industrial material par        excellence.

1-18. (canceled)
 19. An electrolysis device comprising: a cathodeimmersed in an electrolytic solution comprising at least one weak acidand the pH of which is comprised between 4 and 9; an anode immersed inan electrolytic solution capable of forming an electrochemically activebiofilm at the surface of the anode, and comprising at least onebiodegradable organic compound capable of being oxidized at the anode,such that the cathode is or comprises a material selected fromconducting polymers, either oxidized forms or not of Fe, Cr, Ni or Mo,and their different alloys, notably stainless steel, notably 304L, 316L,254 SMO steels.
 20. The device according to claim 19, wherein saidelectrolytic solution in which the cathode is immersed and saidelectrolytic solution in which the anode is immersed, are identical. 21.The device according to claim 19, wherein said electrolytic solution inwhich the cathode is immersed, comprises at least one weak acid with apH comprised between 7 and 8.5.
 22. The device according to claim 19,wherein said electrolytic solution in which the anode is immersed,comprises one or several micro-organisms or consortia of micro-organismsstemming from natural media, known to form electrochemically activebiofilms at the surface of the anode.
 23. The device according to claim22, wherein said micro-organism(s) is/are sown in said electrolytesolution.
 24. The device according to claim 23, wherein said sowing isachieved by adding one or more inocula selected from muds of watereffluents, marine sediments, biofilms, composts or pure micro-organismcultures.
 25. The device according to claim 19, wherein the electrolyticsolution in which the anode is immersed, contains marine sediments orcontains a saline concentration, similar to that of seawater.
 26. Thedevice according to claim 19, wherein said weak acid is selected fromone or more acids from the group comprising orthophosphoric acid,hydrogenphosphate, dihydrogenphosphate, lactic acid, gluconic acidand/or mixtures thereof.
 27. The device according to claim 19, whereinthe concentration of the weak acid is comprised between 0.1 mole perliter and the solubility of said acid in the electrolytic solution inwhich the cathode is immersed.
 28. The device according to claim 19,such that said electrolytic solution in which the cathode is immersedand said electrolytic solution in which the anode is immersed, aredistinct and separated by a separator element between the anodic andcathodic compartments allowing migration of the ions between saidcompartments.
 29. The device according to claim 28 selected from aproton exchange membrane (PEM), a cationic membrane, a ceramic, anultrafiltration (UF) membrane, an anion exchange membrane (AEM), abipolar membrane or further a simple polymeric separator (gas).
 30. Thedevice according to claim 19, wherein said biodegradable organiccompound capable of being oxidized is a natural or synthetic organiccompound or derived from biomass.
 31. The device according to claim 30,wherein the organic compound capable of being oxidized is selected fromthe group comprising acetic acid or an acetate.
 32. The device accordingto claim 19, comprising a means or means for collecting the hydrogenformed.
 33. The device according to claim 19, such that the material ofthe anode is selected from carbon, graphite, stainless steel, nickel,platinum, DSA (dimensionally stable anode).
 34. An electrochemicalmethod for the synthesis of hydrogen comprising the application of adevice according to claim 19, wherein the anode and the cathode areconnected to two opposite terminals of a current or potential source.35. The method according to claim 34, wherein either one or both of theelectrodes are biased beforehand.
 36. An installation comprising adevice according to claim 19.